Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation using a hollow carrier type perforating gun, as an example.
After drilling the various sections of a wellbore that traverses subterranean formations, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a short distance into the formation.
Typically, these perforations are created by detonating a series of shaped charges that are disposed within the casing string and are positioned adjacent to the formation. Specifically, one or more perforating guns are loaded with shaped charges that are connected with a detonator via a detonating cord. The perforating guns are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, coil tubing or other conveyance. Once the perforating guns are properly positioned in the wellbore such that the shaped charges are adjacent to the formation to be perforated, the shaped charges may be detonated, thereby creating the desired hydraulic openings.
The perforating operation may be conducted in an overbalanced pressure condition, wherein the pressure in the wellbore proximate the perforating interval is greater than the pressure in the formation or in an underbalanced pressure condition, wherein the pressure in the wellbore proximate the perforating interval is less than the pressure in the formation. When perforating occurs in an underbalanced pressure condition, formation fluids flow into the wellbore shortly after the perforations are created. This inflow is beneficial as perforating generates debris from the perforating guns, the casing and the cement that may otherwise remain in the perforation tunnels and impair the productivity of the formation. As clean perforations are essential to a good perforating job, perforating in an underbalanced condition is preferred in many instances. It has been found, however, that due to safety concerns, it is desirable to maintain an overbalanced pressure condition during most well completion operations. For example, if the perforating guns were to malfunction and prematurely initiate creating communication paths to a formation, the overbalanced pressure condition will help to prevent any uncontrolled fluid flow to the surface.
To overcome the safety concerns but still obtain the benefits associated with underbalanced perforating, efforts have been made to create a dynamic underbalance condition in the wellbore following charge detonation. The dynamic underbalance is a transient pressure condition created in the wellbore during and immediately following the perforating operation that allows the wellbore to be maintained, for example, at an overbalanced pressure condition prior to perforating. The dynamic underbalance condition can be created using specifically designed surge chambers or simply using hollow carrier type perforating guns. When hollow carrier type perforating guns are used, the interior of the perforating guns contains the shaped charges, the detonating cord and the charge holder tubes. The remaining volume inside the perforating guns consists of air at essentially atmospheric pressure. Upon detonation of the shaped charges, the interior pressure rises to tens of thousands of psi within microseconds. The detonation gases then exit the perforating guns through the holes created by the shaped charge jets and rapidly expand to lower pressure as they are expelled from the perforating guns. The interior of the perforating guns becomes a substantially empty chamber which rapidly fills with the surrounding wellbore fluid. Further, as there is a communication path via the perforation tunnels between the wellbore and the reservoir, formation fluids rush from their region of high pressure in the reservoir through the perforation tunnels and into the region of low pressure within the wellbore and the empty perforating guns. All this action takes place within milliseconds of gun detonation.
While creating a dynamic underbalance is beneficial in many circumstances, it has been found that there are some circumstances where excessive dynamic underbalance causes the perforation tunnels to fail due to, for example, sanding. Also, it has been found that there are some circumstances where insufficient dynamic underbalance fails to fully clean the perforation tunnels. A need has therefore arisen for an improved perforating method that is operable to create effective perforation tunnels that enhance fluid communication between the formation and the wellbore. A need has also arisen for such an improved perforating method that is operable to clean the perforation tunnels without causing damage to the perforation tunnels. Further, a need has arisen for such an improved perforating method that is customizable based upon reservoir conditions.